The invention relates to a method and an apparatus for measuring film thicknesses with the help of a measuring head, which is held with a holding device against the film in such a manner, that the latter is deflected.
Such methods are used particularly for the manufacture of plastic films, so that the film thickness and, optionally, the uniformity of the film thickness over the width of the film can be monitored during the manufacturing process and, if necessary, controlled. For example, in the case of a blown film installation, the measuring head is disposed in such a manner at the film bubble, which is inflated by air blown in and pulled off in the upward direction, that it can move in a circle about the film and, during a revolution, measure the film thickness on the whole periphery of the film bubble. Since it is difficult, particularly in the case of blown film, to dispose the measuring head on both sides of the film, the measuring head should be configured so that the measurement can take place from one side of the film. This can be realized using different measurement principles, for example, with capacitive measurement methods. However, in the case of conductive films, inductive methods also come into consideration and, in the case of transparent films also optical methods.
Capacitive methods are known, for which the sensor is in contact directly with the surface of the film. In the U.S. Pat. No. 5,223,797, a capacitive measuring head is described, which has the shape of a rotatable drum and rolls on the surface of the film. If, as is usually the case, the film moves relative to the sensor, this has the advantage that damage to the film surface resulting from direct contact with the sensor is avoided or, at the very least, decreased. In any case, a certain length of film should lie in contact with the surface of the measuring head, so that a precise and accurate measurement of the thickness becomes possible. For this reason, the measuring head is pressed slightly against the film, so that the film is deflected somewhat at the site of the measuring head. In the case of measurements at a film bubble, the internal pressure in the bubble ensures that the film nestles against the sensor. In the case of measurements at flat film sheets, the deflection of the film ensures that the film lies fully against the surface of the measuring head or, if the measuring head is drum-shaped, is wrapped around the surface of the drum and over a certain length of the periphery.
Measuring heads are also known, which are not in direct contact with the film. Instead, an air cushion is produced between the film and the measuring head, so that the latter hovers a certain distance above the surface of the film. In this case also, the film should be deflected somewhat, so that the defined distance between the surface of the measuring head and the film is retained over a certain length.
Since the internal pressure, in the case of film bubbles, and the tensile stress of the film sheet in the case of flat sheets are subject to certain fluctuations, it is necessary to control the extent of the deflection, that is the depth of immersion of the measuring head in the film. For this purpose, it is known that the distance between the measuring head and the surface of the film can be measured with a measuring device, which is disposed offset to the measuring head. Since this distance varies as a function of the depth of immersion of the measuring head, it is possible to determine and control the depth of immersion indirectly. In the case of a known method, the distance is measured with the help of a scanning flap, which is held at the measuring head and grazes the surface of the film. However, in this connection, it is the disadvantage that, because of friction between the film and the scanning flap, the surface of the film once again may be damaged. In the case of a different method, the distance is measured by means of ultrasound. This method, however, is expensive and relatively inaccurate, since the film produces only a relatively weak echo. Both methods have the disadvantage that the distance measurement is carried out at a position, which is shifted from the actual position of contact between the surface of the film and the film, so that differences in the deformation geometry of the film can lead to inaccuracies.
It is an object of the invention to provide a method and an apparatus, with which an accurate and error-free measurement of the thickness becomes possible by a more precise control of the deflection of the film.
In the case of a method of the type named above, this objective is accomplished owing to the fact that the reaction force, exerted by the film on the measuring head, is measured and controlled by moving the measuring head on a specified nominal value.
Pursuant to the invention, a force measurement is carried out instead of the conventional distance measurement. The reaction force, exerted by the film on the measuring head, depends on the depth of immersion and accordingly enables this depth of immersion to be controlled at a constant value. The advantage consists therein that the force is measured directly with the help of the measuring head at the same place, at which the actual thickness measurement also takes place. A distortion of the measurement results is thus avoided.
Suitable force sensors can be obtained commercially for a large range of forces, so that the measurement of the force can be carried out relatively inexpensively and reliably with standard components. Compared to a conventional ultrasound distance measurement, a decrease in the costs of the equipment is achieved while the accuracy of the measurement is undiminished or even improved. Compared to the use of a scanning flap, there is an important advantage in that additional contact sites between the measuring head and the film are not required for the distance measurement. When a measuring head, which is in the form of a rotatable drum or which hovers on an air cushion, is used, damage to the surface of sensitive film can thus be avoided.
Preferably, the force sensor for measuring the reaction force is disposed between the measuring head and the associated holding device, which can be moved in a direction at right angles to the film with the help of a driving mechanism, such as a stepping motor. The reaction force, measured by the force sensor, is compared with a previously set nominal value and the position of the holding device is controlled with the help of the driving mechanism by means of a comparison between the actual and nominal values.
The nominal value for the reaction force is to be selected so that it corresponds to the desired depth of immersion of the measuring head in the film. The following method is suitable for this purpose. Initially, the measuring head is moved so close to the film, that it (or an air cushion) barely touches the film without deflecting it. This point can also be detected with the help of the force sensor. Subsequently, the holding device is extended further by a defined distance, which corresponds to the desired depth of immersion. With the help of a stepping motor as a driving mechanism, this distance can be set precisely, without requiring the use of an additional device for measuring the distance. When the measuring head has reached its final position, the reaction force, then exerted by the deflected film on the measuring head, is measured and stored as a nominal value. By regulating to this nominal value, the originally set depth of immersion can then be kept constant. The force sensor can be calibrated by a zero measurement before the measuring head contacts the film.